Effect of CWG methylation on expression of plant genes.

The presence of two DNA methyltransferases in Pisum raises the possibility that they serve different functions. In vitro methylation of CWG sequences in the strong cauliflower mosaic virus 35S promoter had no effect on reporter gene expression. In contrast, in vitro methylation of CWG sequences in the relatively weak, CG-deficient Phaseolus vulgaris rbcS2 promoter inhibited transcription. Expression of both constructs was strongly inhibited by extensive CG methylation. A search of published plant promoter sequences revealed that the CG content of promoters is very variable, with some promoters having typical CG islands. In contrast, the distribution of CWG sequences is more even with little evidence for CWG islands.